Q: What kind of carpet do you run on? A: Grey... haha.
I've been asked many times. So here is the real answer.
The carpet many use in Japan is a cheap durable outdoor style carpet. It's thin but strong. You will find it in a DIY store.
It doesn't have any flame retardant so the cars slide on it very well. If it has a chemical impregnation, the cars heat it up and it becomes glue like.
This particular one came in 6 colours and was about $13 a meter for 180cm (6ft) wide. So do the math and build yourself a track.
I just might... one day. Even your garage area would be cheap. About 100$ would do it.